1. Field of the Invention
The present invention relates to a process for producing piezoelectric ceramics. Particularly, the invention relates to a process for producing piezoelectric ceramics which are useful as materials for, for example, piezoelectric ceramic filters, piezoelectric ceramic resonators and other piezoelectric ceramic devices.
2. Description of the Related Art
Piezoelectric ceramics mainly containing lead titanate zirconate (PbTi.sub.x Zr.sub.1-x O.sub.3) or lead titanate (PbTiO.sub.3) have been widely used as piezoelectric ceramics for piezoelectric ceramic filters, piezoelectric ceramic resonators and other piezoelectric ceramic devices.
Piezoelectric ceramics having a layer perovskite structure, such as CaBi.sub.4 Ti.sub.4 O.sub.15 or PbBi.sub.4 Ti.sub.4 O.sub.15 have higher heat resistance and lower loss at high frequency than the piezoelectric ceramics mainly containing lead titanate zirconate or lead titanate. The former piezoelectric ceramics are expected to be materials for piezoelectric resonators for use at high temperatures or at high frequency.
However, piezoelectric ceramics having a layer perovskite structure include highly anisotropic crystals and cannot yield a high electromechanical coefficient if they are produced by conventional processes for producing piezoelectric ceramics.
As possible solutions to this problem, some processes have been proposed. In these processes, the crystallographic axis of a piezoelectric ceramic mainly containing a compound having a layered perovskite crystal structure is preferably oriented in a uniaxial direction by using the crystal anisotropy of the compound to yield a high electromechanical coefficient. For example, T. TAKENAKA et al. reported that a c-axis oriented ceramic made of PbBi.sub.4 Ti.sub.4 O.sub.15 is prepared by hot forging technique and that the resulting ceramic has an electromechanical coefficient k.sub.33 about 1.6 times higher than a sample prepared by a conventional process for producing piezoelectric ceramics (J. Appl. Phys., Vol. 55, No. 4 (1984) P.1092). H. WATANABE et al. reported that a powered Bi.sub.4 Ti.sub.3 O.sub.12 having anisotropic dimensions is prepared by the flux method and the powder having anisotropic dimensions is tape cast to orient the powder, and the grain oriented powder is fired to yield a c-axis oriented ceramic made of Bi.sub.4 Ti.sub.3 O.sub.12 (J. Am. Ceram. Soc., Vol. 72, No. 2 (1989) P.289). T. TANI et al. reported that a c-axis oriented ceramic of CaBi.sub.4 Ti.sub.4 O.sub.15 is prepared by adding powdered TiO.sub.2, Bi.sub.2 O.sub.3 and CaCo.sub.3 to a powdered Bi.sub.4 Ti.sub.3 O.sub.12 having anisotropic dimensions, subjecting the resulting mixture to tape casting to orient the powder having anisotropic dimensions, and firing the grain oriented powder, and that the resulting c-axis oriented ceramic has an electromechanical coefficient k.sub.33 about three times higher than a sample prepared by a conventional process for producing piezoelectric ceramics (Proceedings of Presentations, The 16th Ferroelectric Application Conference, (1999) p.35).
The hot forging technique, however, requires firing with uniaxial pressing in the production of such a piezoelectric ceramic, requires expensive firing furnaces and kilns and exhibits a lower productivity as compared with conventional processes for producing piezoelectric ceramics. The hot forging technique has therefore not yet been in wide practical use.
In the processes for producing oriented ceramics using powders having anisotropic dimensions, impurities are liable to contaminate material powders having anisotropy in the production step. In addition, these processes require a preparation step of an anisotropic powder and thus require complicated production steps as compared with conventional processes for producing piezoelectric ceramics. These processes have therefore not yet been in wide practical use.